Several readers of my WW1 submarine thriller, The Custom of the Trade, have expressed surprise at how difficult it was to detect and sink submarines during the first half of the war. We have been brought up with images from WW2 submarine films of destroyers locking on to the dived submarine and attacking it with depth charges. It comes as a surprise that neither sonar nor depth charges had been invented before 1916.
Prior to 1916, the only means of detecting submarines was visually. Aircraft and airships were the best observation platforms, not only because their height offered visibility at greater ranges, but in clear waters, such as the Mediterranean and Baltic, even dived submarines could be spotted at shallow depths. Early in the war, the Allies set about inventing a means to detect submarines by listening. The work began to come to fruition in 1916. Initially, the hydrophones were lowered into the water, much as practised today by helicopters with dipping sonar. The first success was achieved in April 1916 when UC3 was detected by hydrophone and sunk. The Germans quickly became aware of the new devices and began to develop their own versions later in the year. By mid-1917, the Royal Navy had gone on to test and perfect ASDIC, an active sonar capable not just of detecting submarines by active transmissions, but judging its range and speed by the returning echoes.
The best methods for defending harbours and anchorages from submarine attack were nets at mines. Several submarines from all nations were lost to mines throughout the war. In 1915, the Turks and Germans managed to lower mines down to set depths to explode alongside the hulls of submarines stuck in their anti-submarine nets. Otherwise, once submarines were detected, the preferred offensive methods of attack were shellfire or ramming. Obviously, these were ineffective against dived submarines. Other submarines could fire a torpedo at an enemy submarine at periscope depth and attempts were made by aircraft and airships to bomb submarines on the surface or in the process of diving.
Even before the war, the Royal Navy had begun work on a weapon that could be used against submarines at depth, but it was not until January 1916 that an effective device, that we now know as the depth charge, was deployed in RN ships. In March 1916, a German U-boat was sunk by depth charges dropped from a Q-ship. The Germans were again not slow to catch up with the new technology, but did not employ depth charges in numbers before 1917.
It must not be overlooked that in the same way as ships were not able to detect the noise of dived submarines until 1916, submarines were almost as deaf. A ship’s engine and propeller noises could be heard through the hull of a submarine, but the only means of identifying the target’s speed, course and range was visually. Raising a periscope risked counter-detection. Towards the end of the war, submarines were fitted with hydrophones that could be lowered through the hull, but it was not until the early 1920s that trials were conducted into the use of ASDIC or active sonar in submarines. Only then could the concept of blind attacks, ie by sound only, be developed.